B. Equilibria at Higher Temperatures. Formation of Double Salts.—After having studied the relationships which are found in the neighbourhood of the freezing points of the components, we now pass to the discussion of the equilibria which are met with at higher temperatures. In this connection we shall confine the discussion entirely to the systems formed of two salts and water, dealing more particularly with those cases in which the water is present in relatively large amount and acts as solvent. Further, in studying these systems, one restriction must be made, viz. that the single salts are salts either of the same base or of the same acid; or are, in other words, capable of yielding a common ion in solution. Such a restriction is necessary, because otherwise the system would be one not of three but of four components.[[336]]
Transition Point.—As is very well known, there exist a number of hydrated salts which, on being heated, undergo apparent partial fusion; and in Chapter V. the behaviour of such hydrates was more fully studied in the light of the Phase Rule. Glauber's salt, or sodium sulphate decahydrate, for example, on being heated to a temperature of about 32.5°, partially liquefies, owing to the fact that the water of crystallization is split off and anhydrous sodium sulphate formed, as shown by the equation—
Na2SO4,10H2O = Na2SO4 + 10H2O
The temperature of 32.5°, it was learned, constituted a transition point for the decahydrate and anhydrous salt plus water; decomposition of the hydrated salt occurring above this temperature, combination of the anhydrous salt and water below it.
Analogous phenomena are met with in systems constituted of two salts and water in which the formation of double salts can take place. Thus, for example, if d-sodium potassium
tartrate is heated to above 55°, apparent partial fusion occurs, and the two single salts, d-sodium tartrate and d-potassium tartrate, are deposited, the change which occurs being represented by the equation—
4NaKC4O6H4,4H2O = 2Na2C4O6H4,2H2O + 2K2C4O6H4,½H2O + 11H2O
On the other hand, if sodium and potassium tartrates are mixed with water in the proportions shown on the right side of the equation, the system will remain partially liquid so long as the temperature is maintained above 55° (in a closed vessel to prevent loss of water), but on allowing the temperature to fall below this point, complete solidification will ensue, owing to the formation of the hydrated double salt. Below 55°, therefore, the hydrated double salt is the stable system, while above this temperature the two single salts plus saturated solution are stable.[[337]]
A similar behaviour is found in the case of the double salt copper dipotassium chloride (CuCl2,2KCl,2H2O or CuK2Cl4,2H2O).[[338]] When this salt is heated to 92°, partial liquefaction occurs, and the original blue plate-shaped crystals give place to brown crystalline needles and white cubes; while on allowing the temperature to fall, re-formation of the blue double salt ensues. The temperature 92° is, therefore, a transition point at which the reversible reaction—
CuK2Cl4,2H2O